This page has the numbers for basil: 10 inches between plants with 18 inches between rows, four to a square foot, started indoors six weeks before your last frost and set out a week after it, an inch of water a week, and a quarter to three quarters of a pound per plant, with the first cutting 30 to 60 days after sowing.
The figure that actually decides the crop is a temperature, not a spacing: basil is damaged below 50F, which is why a plant set out early never recovers. After that the page is about pinching — the technique that turns one stem into two, and a plant that produces for a summer into one that flowers and finishes in July.
Warmth first
Basil is a tender annual from a hot climate and it does not pretend otherwise. It suffers below 50F, blackening at the leaf edges, and a plant set out too early never really recovers.
Sow indoors six weeks before your last frost date and plant out a week after it, once nights are reliably mild. Direct sowing works too, from about a week after the last frost. Basil germinates quickly in warm soil and slowly in cool.
Give it the warmest, sunniest spot you have. A south-facing wall or a patio that holds heat suits it better than an open bed.
Spacing
Ten inches between plants, 18 inches between rows, or four to a square foot. That seems close for a plant that gets bushy, and it is deliberate — basil grown with a little company stays upright. Too crowded, though, and the still humid air between plants invites downy mildew, which is now the most common serious basil problem.
Water and food
An inch a week. Basil likes steady moisture and hates sitting wet, which is a real distinction: a plant in badly drained soil or a pot without a drainage hole will wilt and collapse even though the soil is damp.
Water the soil in the morning, so any splash on the leaves dries during the day.
Feed sparingly, which is counter-intuitive for a leaf crop. The aromatic oils that make basil worth growing are diluted by lush, nitrogen-driven growth. Compost worked into the bed before planting is usually all it needs for a season in the ground.
Pinching, which is the whole technique
This is what separates a good basil plant from a disappointing one.
Start when the plant has three or four pairs of leaves: pinch out the growing tip just above a leaf pair. The plant responds by sending out two shoots from that joint. Do the same to each of those once they have grown, and the plant doubles its branches each time.
Then keep pinching out flower buds the moment they appear, taking the pair of leaves beneath them as well. A flowering basil redirects everything into seed and the leaves go small and sharp-tasting.
Harvest the same way — cut whole stems above a leaf pair rather than picking individual leaves. Every harvest is also a pinch.
Companions and containers
Basil is traditionally planted with tomatoes, and while the pest-repelling claims are folklore rather than science, the two genuinely want the same conditions and get harvested together, so the pairing is practical.
It grows very well in a pot — an eight to ten inch container suits one plant, and a window box takes three. Container basil needs watering more often and benefits from an occasional weak feed, since a small volume of mix runs out of nutrients within weeks.
Ending the season
Basil will not survive frost and does not overwinter outdoors. Before the first frost, cut the whole plant and use it: pesto freezes well, and basil dries poorly compared with woodier herbs. Cuttings root readily in water if you want a plant on the windowsill through autumn.